Mystery prehistoric fossil verified as giant fungus

This is what Prototaxites might have looked like in their prehistoric landscape (Illustration: Elsevier/Hueber)

Francis Hueber peers up at a Prototaxite fossil

(Image: Carol Hotton)

Scientists have identified the Godzilla of fungi – a giant, prehistoric fossil that has evaded classification for more than a century.

A chemical analysis has shown that the 6-metre-tall organism with a tree-like trunk was a fungus that became extinct more than 350 million years ago.

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Known as Prototaxites, the giant fungus has intrigued scientists, who originally thought it was a conifer. In 2001, Francis Hueber at the National Museum of Natural History in Washington DC, US, first suggested it may be a fungus, based on an analysis of the fossil’s internal structure. But he had no conclusive proof.

“No matter what argument you put forth, people say it’s crazy,” says C. Kevin Boyce, a geophysicist at the University of Chicago in Illinois, US. “A 6-metre-fungus doesn’t make any sense, but here’s the fossil.”

Boyce, who says Prototaxites “get his vote for being one of the weirdest organisms that ever lived”, helped solve the mystery by comparing the types of carbon found in the giant fossil with plants that lived about the same time, some 400 million years ago.

Closer cousin

Plants only absorb carbon from carbon dioxide in the air around them. As a result, the ratio of two carbon isotopes found in atmospheric CO2 – carbon-12 and carbon-13 – is the same for all plants that live during the same period.

“If you are an animal, you will look like whatever you eat,” explains Boyce. Previously classified as plants, fungi are now considered a closer cousin to animals, although they chemically absorb their food rather than eat it mechanically.

The researchers found that the carbon ratios from the different fossil samples of the fungi differed much more than would have been expected of a plant from that era, strengthening the evidence that Prototaxites is a fungus that absorbed some of its carbon from sources other than the air.

Samples of the giant fungi have been found all over the world since its discovery a century ago. It lived between 420 million and 350 million years ago, at a time when millipedes and worms were among the first creatures to make their home on dry land. No animals with a backbone had left the oceans.

“A 6-metre fungus would be odd enough in the modern world, but at least we are used to trees quite a bit bigger,” says Boyce. “Plants at that time were a few feet tall, invertebrate animals were small, and there were no terrestrial vertebrates. This fossil would have been all the more striking in such a diminutive landscape.”